Implement lazy file loading for Feeds

Implement lazy file loading for Feeds.
Which means the following function:

/**
 * Implements hook_init().
 */
function references_init() {
  // Include feeds.module integration.
  if (module_exists('feeds')) {
    module_load_include('inc','references','references.feeds');
  }
}

is not needed at all in 7.x
This would improve some performance for every request.

See: #588766: Standardize lazy-loading of optional include files